Abstract. HUmic-LIke Substances (HULIS) have been identified as major
contributors to the organic carbon in atmospheric aerosol. The term
"HULIS" is used to describe the organic material found in aerosol
particles that resembles the humic organic material in rivers and
sea water and in soils. In this study, two sets of filter samples
from atmospheric aerosols were collected at different sites. One set
of samples was collected at the K-puszta rural site in Hungary,
about 80 km SE of Budapest, and a second was collected at a site in
Rondônia, Amazonia, Brazil, during the Large-Scale
Biosphere-Atmosphere Experiment in Amazonia – Smoke Aerosols,
Clouds, Rainfall and Climate (LBA-SMOCC) biomass burning season
experiment. HULIS were extracted from the samples and their
hygroscopic properties were studied using a Hygroscopicity Tandem
Differential Mobility Analyzer (H-TDMA) at relative humidity (RH)
<100%, and a cloud condensation nucleus counter (CCNC) at RH
>100%. The H-TDMA measurements were carried out at a dry diameter
of 100 nm and for RH ranging from 30 to 98%. At 90% RH the HULIS
samples showed diameter growth factors between 1.04 and 1.07,
reaching values of 1.4 at 98% RH. The cloud nucleating properties
of the two sets of aerosol samples were analysed using two types of
thermal static cloud condensation nucleus counters. Two different
parameterization models were applied to investigate the potential
effect of HULIS surface activity, both yielding similar results. For
the K-puszta winter HULIS sample, the surface tension at the point
of activation was estimated to be lowered by between 34%
(47.7 mN/m) and 31% (50.3 mN/m) for dry sizes between 50 and
120 nm in comparison to pure water. A moderate lowering was also
observed for the entire water soluble aerosol sample, including both
organic and inorganic compounds, where the surface tension was
decreased by between 2% (71.2 mN/m) and 13% (63.3 mN/m).